1. Field of the Invention
This invention relates to the field of high impedance elements, and particularly to circuits and methods for providing on-chip high impedance elements in a limited die area.
2. Description of the Related Art
Analog integrated circuits (ICs) often have a need for a high impedance element. For example, many filter designs require an R-C network, the components of which are sized to provide a desired time constant. However, when a large time constant is needed, the size of one or more of the components may exceed the available on-chip area, thereby requiring the use of an external capacitor and/or resistor. The use of off-chip components is undesirable, however, due to their added cost and space requirements.
The number of on-chip elements available to an IC designer typically depends on the IC's particular fabrication process. For example, a particular “HBT” process—which employs a compound semiconductor material such as indium phosphide (InP) to fabricate heterojunction bipolar transistors (HBTs)—may provide transistors, capacitors and thin film resistors. The on-chip capacitors are typically limited to tens of pF, while the on-chip resistors would require a very large die area to realize a resistance in the mega-ohms. Also, due to their large area, such resistors can have a significant parasitic capacitance which can degrade circuit performance. As such, the providing of a time constant on the order of milliseconds using on-chip elements may be impractical or impossible—especially for certain fabrication processes which offer a limited selection of elements.